Rotary helical splined actuators have been employed in the past to achieve the advantage of high-torque output from a simple linear piston-and-cylinder drive arrangement. The actuator typically uses a cylindrical body with an elongated rotary output shaft extending coaxially within the body, with an end portion of the shaft providing the drive output. An elongated annular piston sleeve is disposed between the body and the shaft and coaxially receives the shaft therein. The piston sleeve has a sleeve portion splined to cooperate with corresponding splines on the body interior and the output shaft exterior. The piston sleeve is reciprocally mounted within the body and has a head for the application of fluid pressure to one or the other opposing sides thereof to produce axial movement of the piston sleeve.
As the piston sleeve linearly reciprocates in an axial direction within the body, the outer splines of the sleeve portion engage the splines of the body to cause rotation of the sleeve portion. The resulting linear and rotational movement of the sleeve portion is transmitted through the inner splines of the sleeve portion to the splines of the shaft to cause the shaft to rotate. Bearings are typically supplied to rotatably support one or both ends of the shaft relative to the body.
While such an arrangement produces a relatively high-torque output, the capability of the actuator to support high moment loads and large axial thrust loads has been limited. The actuator typically has a slender shaft with bearings between the shaft and end flanges or end caps of the body, with the bearings positioned radially inward of the body sidewall. It is desirable to use rotary actuators to rotate platforms carrying heavy loads, such as a large diameter platform which extends radially far beyond the actuator body and which carries a crane, bucket lift or other mechanism having a boom reaching far outward of the platform. One such arrangement is shown in the inventor's U.S. Pat. No. 4,508,016.
The actuator body is vertically oriented and attached to the frame of the vehicle or other structure carrying the platform, and the actuator shaft is attached to the platform to cause its rotation. In the aforementioned patent, the platform weight is supported by the actuator body rather than the shaft, but in many situations it is desirable to have the actuator shaft directly support the platform. The conventional actuator is not well constructed to handle the high moments encountered when the shaft centrally supports a platform since it does so in an almost needle point balanced arrangement. In such an arrangement, when the boom of the device carried by the platform is extended, the moments become extremely large and difficult for the conventional actuator shaft and shaft bearing configuration to handle. Further, the axial thrust loads encountered due to the weight of the platform, the crane or other mechanism mounted thereon and the workload it carries are far too great for the conventional actuator shaft bearing configurations. Other substantially vertical orientations of the actuator are envisioned which also subject the actuator shaft to high moments and large axial thrust loads, such as used to steerably turn the wheel assembly of a vehicle while supporting the weight of the vehicle above the wheel assembly.
The long length of prior art rotary actuators resulting from the conventional actuator construction used has also been a limitation to use of such actuators to rotate platforms, as well as to their use in other situations requiring a shorter package to fit within the size constraints of the equipment design with which the actuator is to be used.
Another problem involves the backlash encountered in the torque-transmitting elements of the actuator as the piston sleeve moves from one axial direction to the other in response to the application of fluid pressure. While accurate machining will reduce the backlash problem, this procedure substantially increases the manufacturing cost. Even with accurate machining, conventional machining techniques are virtually incapable of totally eliminating the slack which produces the backlash problem. Furthermore, to the extent more accurate tolerances produce actuator parts which fit tightly together and reduce slack, assembly of the actuator becomes difficult. While accurate machining reduces slack initially, should the splined parts wear during usage or otherwise lose their original tolerances, no means exists for elimination of the slack that develops without disassembly of the actuator and possible remachining or replacement of the parts. Backlash is a particular problem when the actuator is used to rotate a crane or bucket lift platform since the backlash in the actuator tends to be magnified at the tip of the extended boom, reducing the control the operator has over the positioning of the boom tip and the stability of the boom as the piston sleeve moves from one axial direction to the other.
It will therefore be appreciated that there has long been a significant need for fluid-power bearing actuators and devices which are capable of handling increased moments and axial shaft loads. The device should have a reduced overall length and have means to eliminate backlash. The device should not require exceptionally accurate machining of the torque-transmitting parts to eliminate the slack that produces backlash. The actuator should be easy to assemble and provide means for eliminating the slack causing the backlash problem after the actuator is assembled. Elimination of the slack should be accomplished in a simple manner without requiring disassembly of the torque-transmitting parts from the body. The present invention fulfills these needs and further provides other related advantages.